Snake III

Review — Snake II 5 min

• Apples land on a random cell, rolled again while the cell is on the snake.
• Eating skips the delete, so the snake grows.
• A crash is the new head found in the body, from item 2, after the tail has moved.

Quick-fire

Your loop has wait (0.2) seconds and nothing else slow. How long do 30 loops take?

Today’s Topic 3 min

• A tick driven by the timer, not by counting waits
• Speed that grows with the snake’s length, with a limit
• Walls typed as rows of letters — Tile Maker’s format — stamped once
• Wrap-around with mod

Learning outcome

By the end of this lesson you will be able to:

1. Keep a steady beat with the timer.
2. Read a level from a list of text and ask what is in any cell.
3. Redraw only what changed, instead of everything.

Learnt — time, speed and walls 28 min

1 · A tick on the timer

wait (0.2) seconds waits 0.2 seconds on top of everything else the loop does. Instead, keep the time of the next tick in next, and step whenever the timer passes it.

when flag clicked
reset timer
set [next v] to (0)
forever
  wait until <(timer) > (next)>
  change [next v] by (gap)
  step :: custom
  draw changes :: custom
end
            

2 · Faster as it grows

Work out gap from the length after every tick. Start at 0.25 s, take 0.01 off for every cell past 3 — but never go below 0.08, or nobody could steer.

set [gap v] to ((0.25) - ((0.01) * ((length of [xs v]) - (3))))
if <(gap) < (0.08)> then
  set [gap v] to (0.08)
end
            

3 · Walls are letters

A level is a list called level: 12 items, one per row, each 16 letters long. . is grass and # is a wall — exactly like Tile Maker. A Wall sprite stamps every # once, at the start.

define tile at (c) (r)
set [tile v] to (letter (c) of (item (r) of [level v]))
            

define build walls
set [wr v] to (1)
repeat (12)
  set [wc v] to (1)
  repeat (16)
    if <(letter (wc) of (item (wr) of [level v])) = [#]> then
      go to cell (wc) (wr) :: custom
      stamp
    end
    change [wc v] by (1)
  end
  change [wr v] by (1)
end
            

The fix is to stop redrawing everything. Each tick only two cells change: the old head becomes body, and the old tail becomes grass again. Our backdrop has a cell of each green as a costume (grass light where c + r is even, grass dark where it is odd).

define draw changes
if <(length of [xs v]) > (1)> then
  switch costume to [body v]
  go to cell (item (2) of [xs v]) (item (2) of [ys v]) :: custom
  stamp
end
switch costume to [head v]
point in direction (dir)
go to cell (item (1) of [xs v]) (item (1) of [ys v]) :: custom
            

4 · Wrap-around with mod

Lesson 41’s wrap needed four ifs. mod does it in one line per list: it is the remainder after dividing, and in Scratch it is never negative.

set [c v] to ((((c) - (1)) mod (16)) + (1))
set [r v] to ((((r) - (1)) mod (12)) + (1))
            

Basic — a walled garden that speeds up 25 min

Open your snake from Lesson 42.

Step 1 — the level and the walls

1. Make a list level and type in 12 rows: a full row of # top and bottom, and # at both ends of every row between.
2. Add a Wall sprite with wall.svg. Give it build walls, run when it receives build.

Step 2 — draw only what changes

1. Upload grass-light.svg and grass-dark.svg to the Snake.
2. Build draw changes, and paint the tail back to grass in step.
3. At the green flag: erase all, then broadcast [build] and wait, then draw the whole snake once.

Step 3 — the timer and the walls

1. Swap the wait in the loop for the timer tick from Learnt section 1.
2. Work out gap after every tick (Learnt section 2).
3. Add tile at. After step: if tile is #, say Hit a wall! and stop.
4. Keep apples off the walls: pick columns 2–15 and rows 2–11.

Levels

Five apples on the open garden, then level 2: the pillars.

• Keep a level no variable.
• When the score reaches 5 on level 1: fill level with level 2’s rows and build the walls.
• Set next to the timer afterwards, so the pause for the new level does not make it race.

if <<(score) = (5)> and <(level no) = (1)>> then
  set [level no v] to (2)
  delete all of [level v]
  add [................] to [level v]
  add [................] to [level v]
  add [...##......##...] to [level v]
  say [Level 2!] for (1) seconds
  set [next v] to (timer)
end
            

Portals

Two O letters in the level are a pair of portals. Steer into one and come out of the other, still going the same way.

• Upload portal.svg to the Wall sprite and stamp it for every O.
• In step: if the new head’s tile is O, move it to the other portal, then one more cell on.

tile at (c) (r) :: custom
if <(tile) = [O]> then
  if <<(c) = (2)> and <(r) = (2)>> then
    set [c v] to (15)
    set [r v] to (11)
  else
    set [c v] to (2)
    set [r v] to (2)
  end
  change [c v] by (dx)
  change [r v] by (dy)
end
            

Pause

Press p to pause. The catch: the timer does not stop. Unpause and the loop sees it is two seconds behind — and races through ten ticks at once.

• While paused, the loop waits.
• When it carries on, push next on by however long the pause lasted.

when [p v] key pressed
if <(paused) = (0)> then
  set [paused v] to (1)
  set [paused at v] to (timer)
else
  set [paused v] to (0)
  change [next v] by ((timer) - (paused at))
end
            

Draw your own walls

Tile Maker’s editor, for Snake: click a cell to add or remove a wall, then press space to play.

• Turn the mouse into a cell: floor((mouse x + 240) / 30) + 1 and floor((180 − mouse y) / 30) + 1.
• A click rebuilds that row letter by letter, swapping the clicked letter between . and #.
• Then erase all and build the walls again.

Summary 5 min

• A wait in a loop drifts. A tick on the timer keeps a steady beat.
• Speed comes from a sum on the length, with a floor so it never gets impossible.
• A level is a list of text. letter (c) of (item (r) of level) tells you what is in any cell.
• Redraw only what changed — the head and the tail — and stamped walls survive.
• mod wraps a number round, in one block.

Timer

Scratch’s stopwatch: seconds since it was last reset.

Drift

A small delay that adds up, tick after tick, until the game is noticeably slow.

mod

The remainder after dividing. 17 mod 16 is 1.

Hand in

Save your game. Next lesson a second snake joins — on the same keyboard — and the hardest question is who crashed first.